Printed Circuit Assembly And A Touch Sensitive System Comprising The Assembly

ABSTRACT

The disclosure relates to a printed circuit assembly, PCA, comprising: a printed circuit board, PCB; a group of components comprising an emitter, a detector and an integrated circuit, IC wherein the components are electrically bonded to the PCB, and the emitter and the detector are electrically connected to the integrated circuit via the PCB, the integrated circuit is further configured to control operation of the emitter and the detector in the same group, and a first coating covering at least one of the components in the group, wherein the first coating is made of an optically transparent material. The disclosure also relates to a method for fabricating a printed circuit assembly, and a touch sensitive system comprising the printed circuit assembly.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of Swedish patent applicationNo. 1350460-0, filed 11 Apr. 2013.

FIELD OF THE INVENTION

The present invention relates to a printed circuit assembly, and use ofsuch assembly in a touch sensitive system according to the preamble ofthe independent claims.

BACKGROUND OF THE INVENTION

Touch sensing systems (“touch systems”) are in widespread use in avariety of applications. Typically, the touch systems are actuated by atouch object such as a finger or stylus, either in direct contact, orthrough proximity (i.e. without contact), with a touch surface. Touchsystems are for example used as touch pads of laptop computers, incontrol panels, and as overlays to displays on e.g. hand held devices,such as mobile telephones. A touch panel that is overlaid on orintegrated in a display is also denoted a “touch screen”. Many otherapplications are known in the art. To an increasing extent, touchsystems are designed to he able to detect two or more touchessimultaneously, this capability often being referred to as “multi-touch”in the art.

WO2011/028169 and WO2011/049512 disclose multi-touch systems that arebased on frustrated total internal reflection (FTIR). Light sheets arecoupled into a panel by emitters to propagate inside the panel by totalinternal reflection (FIR). When an object comes into contact with atouch surface of the panel, the propagating light is attenuated at thepoint of touch. The transmitted light is measured at a plurality ofoutcoupling points by one or more light detectors. The signals from thelight detectors are processed for input into an image reconstructionalgorithm that generates a 2D representation of interaction across thetouch surface. This enables repeated determination of currentposition/size/shape of touches in the 2D representation while one ormore users interact with the touch surface. Examples of such touchsystems are found in e.g. U.S. Pat. No. 3,673,327, U.S. Pat. No.4,254,333 and U.S. Pat. No. 6,972,753.

The emitters and detectors of the system should be connected to externalelectrical circuitry. These components are typically integrated into andelectrically connected to a printed circuit board, a PCB, and placedalong the periphery of the touch panel.

Different solutions exist to integrate the components to the PCB. Onecategory of solutions makes use of wire bonding to connect the componentto external circuitry. One solution in this category use FR-4, acomposite material composed of woven fiberglass cloth. A thin layer ofcopper foil is laminated to one, or both sides of an FR-4 glass epoxypanel. These are commonly referred to as “copper-clad laminates”. Acomponent is connected via bonding wires to a FR-4 panel and forms witha molding a substrate package. The substrate package is in turnconnected to the PCB via connections on the package.

Another solution in this category makes use of a lead frame package,where the component is connected via bonding wires to a lead frame. Thecomponent and bonding wires are protected by a molding. The lead frameis in turn connected to the PCB.

Another category of solutions for connecting components to externalcircuitry is to use flip chip, also known as controlled collapse chipconnection, or its acronym, C4. The solution makes use of solder bumpsthat has been deposited to the external circuitry. In order to mount thechip to external circuitry, e.g. a PCB, it is flipped over so that itstop side faces down, and aligned so that its pads align with matchingpads on the external circuit, and then the solder is flowed to completethe interconnect.

The components and its wire bonding, if used, can be protected by socalled glob-top coating. Glob-top is a coating consisting of a drop ofspecially formulated resin deposited over the chip and its wire bonds,to provide mechanical support and exclude contaminations such asfingerprint residues which could disrupt circuit operation.

The cost of each component, i.e. an emitter, detector or integratedcircuit, is low compared to the cost of the rest of the package it is apart of It is also a time consuming process to first connect eachcomponent to its package, and secondly connect the package to theexternal circuitry, e.g., the PCB. If the PCB is a flexible film,connections between the package and the PCB may be exerted to mechanicalstress if the flexible film is e.g. bent.

It is thus an object of the invention to reduce the cost for the systemand to reduce the time for mounting the components. It is a furtherobject to provide an incoupling solution to the system.

SUMMARY OF THE INVENTION

According to a first aspect, the object is achieved by a printed circuitassembly, PCA comprising a printed circuit board, PCB, a group ofcomponents comprising an emitter, a detector and an integrated circuit,IC, wherein the components are electrically bonded to the PCB, and theemitter and the detector are electrically connected to the integratedcircuit via the PCB. The integrated circuit is further configured tocontrol operation of the emitter and the detector in the same group. ThePCA further comprises a first coating covering at least one of thecomponents in the group, where the first coating is made of an opticallytransparent material.

With a printed circuit assembly according to the invention, there is noneed for any packaging of the components before they are bonded to thePCB. Further, the components are placed at predefined positions on thePCB such that the PCA can be immediately used in a touch sensitivesystem essentially without any time consuming positioning of each groupof component to the system. The number of connections between thecomponents and the PCB may be larger than if a package is used, andmechanical stress exerted on the PCA will be distributed on a largernumber of connections making the PCA less vulnerable to the stress.

According to one embodiment, the optically transparent material istransparent to infra-red light.

According to one embodiment, the optically transparent material isconfigured to block visible light.

According to one embodiment , the first coating has a box-shaped formwith two longitudinal side surfaces and a top surface, wherein at leastone of the side surfaces and top surface is a substantially planarsurface.

According to one embodiment, at least one of the components in the groupare electrically bonded to the PCB via one or several wires from eachcomponent to the PCB.

According to one embodiment, the integrated circuit is covered with asecond coating, wherein the second coating comprises an opticallynon-transparent material.

According to one embodiment, the PCB is a flexible printed circuitboard.

According to one embodiment, the PCB has a longitudinal extension andcomprises a plurality of groups of components, wherein the groups ofcomponents are positioned at a distance from each other along thelongitudinal extension of the PCB.

According to a second aspect, at least part of the object is achieved bya touch sensitive system comprising a touch sensitive panel defining atouch surface and a printed circuit assembly according to any of theprinted circuit assembly embodiments as described herein, wherein theprinted circuit assembly is attached to the touch sensitive panel alongthe periphery of the touch surface, and a control unit connected to theintegrated circuit in each group, and configured to control operation ofthe components in each group.

According to one embodiment, the touch sensitive system is based onFrustrated Total Internal Reflection, FTIR.

According to a third aspect, at least part of the object is achieved bya method for fabricating a printed circuit assembly, comprising

arranging at least one group of components comprising an emitter, adetector and an integrated circuit, IC, on a printed circuit board, PCB;

bonding the components of the at least one group to the PCB, such thatthe components are electrically bonded to the PCB, and the emitter andthe detector are electrically connected to the integrated circuit viathe PCB;

forming a first coating covering at least one of the components, whereinthe first coating is made of an optically transparent material;

curing the first coating.

According to one embodiment, the step of forming a first coatingcomprises forming a first coating having a box-shaped form with twolongitudinal side surfaces and a top surface, wherein at least one ofthe side surfaces and top surface is a substantially planar surface.

According to one embodiment, the curing step comprises eitherultraviolet curing or heat curing.

Preferred embodiments are set forth in the dependent claims and in thedetailed description.

SHORT DESCRIPTION OF THE APPENDED DRAWINGS

Below the invention will be described in detail with reference to theappended figures, of which:

FIG. 1 illustrates a side view of a touch arrangement based on FTIR.

FIG. 2 illustrates a top view of the touch arrangement in FIG. 1.

FIG. 3 illustrates a printed circuit assembly according to someembodiments of the invention.

FIGS. 4A and 4B illustrates different attachment alternatives of theprinted circuit assembly in FIG. 3.

FIG. 5 illustrates a touch sensitive system according to someembodiments of the invention.

FIG. 6 illustrates a method for fabricating a printed circuit boardaccording to some embodiments of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

FIGS. 1 and 2 illustrates a side view and a top view of an exampleembodiment of a touch arrangement 1 to be used in a touch-sensitivesystem 11 that is based on the concept of FTIR (Frustrated TotalInternal Reflection), also denoted “FTIR system”. The touch arrangement1 operates by transmitting light inside a touch sensitive panel 2, fromlight emitters 4 to light sensors or detectors 5, so as to illuminate atouch surface 3 from within the panel 2. The panel 2 is made of solidmaterial in one or more layers and may have any shape. The panel 2defines an internal radiation propagation channel, in which lightpropagates by internal reflections. In the example of FIG. 1, thepropagation channel is defined between two boundary surfaces 6, 7 of thepanel 2, thus a top surface 6 and a bottom surface 7, where the topsurface 6 allows the propagating light to interact with touching objects27, 9 and thereby defines the touch surface 3. This is achieved byinjecting the light into the panel 2 such that the light is reflected bytotal internal reflection (TIR) in the touch surface 3 as it propagatesthrough the panel 2. The light may be reflected by TIR in the bottomsurface 7 or against a reflective coating thereon. It is alsoconceivable that the propagation channel is spaced from the bottomsurface 7, e.g. the panel 2 comprises multiple layers of differentmaterials. The panel 2 may be designed to be overlaid on or integratedinto a display device or monitor.

The arrangement 1 allows one or several objects 27, 9 that is broughtinto close vicinity of, or in contact with, the touch surface 3 tointeract with the propagating light at the point of touch. In thisinteraction, part of the light may be scattered by the object 27, 9,part of the light may be absorbed by the object 27, 9 and part of thelight may continue to propagate in its original direction across thepanel 2. Thus, the touching object 27, 9 causes a local frustration ofthe total internal reflection, which leads to a decrease in the energy(power/intensity) of the transmitted light, as indicated by the thinnedlines downstream of the touching objects 27, 9 in FIG. 1. If two objects27 and 9 happen to be placed after each other along a light path i froman emitter 4 to a detector 5, part of the light will interact with boththese objects 27, 9. Provided that the light energy is sufficient, aremainder of the light will interact with both objects 27, 9 andgenerate an output signal that allows both interactions (touch inputs)to be identified. The output signal is received to a control unit 10which processes the output signal to detect interaction with thetouching object(s) 27, 9.

As illustrated in FIG. 2, the emitters 4 are distributed along theperimeter of the touch surface 3 of the panel 2. The emitters 4 generatea corresponding number of light sheets inside the panel 2. Each emitter4 generates a beam of light that expands in the plane of the panel 2while propagating in the panel 2. Each beam propagates from one or moreentry or incoupling points on the panel 2. The detectors 5 aredistributed along the perimeter of the touch surface 3 of the panel 2 toreceive the light from the emitters 4 at a number of spaced-apartoutcoupling points on the panel 2. The emitters 4 and detectors 5 arehere grouped in couples with one emitter 4 and one detector S side byside, but the distribution could be different. For example, two or threeemitters 4 located side by side, and two or three detectors 5 locatedside by side could be alternately distributed along the perimeter of thetouch surface 3. A light path from an emitter 4 to a detector 5 isdefined as a detection line 8. In the figure the detection lines 8 fromone emitter 4 to a plurality of detectors 5 are denoted as dotted lines.The plurality of detectors 5 are shaded to indicate their participationin receiving light at ends of the detection lines 8.

In FIG. 3 an example of a printed circuit assembly, PCA, 15 is shown.The PCA 15 comprises a printed circuit board, PCB, 17, and at least onegroup 13 of components comprising an emitter 4, a detector 5 and anintegrated circuit, IC, 12. In FIG. 3 three groups 13 of components areillustrated, but it is understood that the number can be more or less.The PCB 17 may have a longitudinal extension and comprise a plurality ofgroups 13 of components. The groups 13 of components are then positionedat a distance from each other along the longitudinal extension of thePCB 17. The PCB 17 may he a flexible printed circuit board. The term“flexible” means that the assembled PCB 17, thus the PCA 15, will beable to bend without breaking. A suitable material for the flexibleprinted circuit board is e.g. flexible plastic substrates such aspolyimide, polymer thermoplastics such as PEEK (Polyether ether ketone),or polyester film. The PCA 15 may be used with the arrangement 1 inFIGS. 1 and 2 to inject light into the panel 2 and to detect the same.One or several of the components may be electrically bonded to the PCB17 via wire bonding where one or several wires 21 from the componentsare connected to the PCB 17 via pads. Alternatively, one or more of thecomponents may be electrically bonded to the PCB 17 via so called flipchip bonding, via matching soldering pads on the PCB 17 and thecomponents). In any of the alternatives, the components are connecteddirectly to the PCB 17 without any intermediate attachment means. Theemitter 4 and the detector 5 are electrically connected to theintegrated circuit 12 via the PCB 17. The integrated circuit 12 isconfigured to control operation of the emitter 4 and the detector 5 inthe same group 13. The PCB 17 is thus used to mechanically support thecomponents and to electrically connect the components. To electricallyconnect the components, the PCB 17 may comprise conductive pathways,tracks or signal traces e.g. etched from copper sheets laminated onto anon-conductive substrate.

The PCA 15 further comprises a first coating 16 covering at least one ofthe components in the group 13. The first coating 16 is made of anoptically transparent material, such that the emitter 4 can emit lightinto the panel 2 via the first coating 16, and the detector 5 can detectlight propagating in the panel 2 via the first coating 16. The opticallytransparent material is preferably transparent to near infra-red light,thus, it lets through electromagnetic radiation with the wavelengthsfrom 0.74 μm to 1.4 μm. Example of such materials are e.g. a polymer,epoxy, etc. The first coating 16 may additionally comprise a materialthat is blocking visible light.

The integrated circuit 12 may be covered with a second coating 22. Thesecond coating 22 may be located between the first coating 16 and theintegrated circuit 12 as illustrated in FIG. 3. Alternatively, theintegrated circuit 12 is only covered by the second coating 22, and thusnot with a first coating 16. The second coating 22 is only illustratedon one of the integrated circuits 12 on the PCA 15, but it is understoodthat all the integrated circuits 12 on the PCA 15 may be covered with asecond coating 22. The second coating 22 comprises an opticallynon-transparent material, such that the integrated circuit 12 will beprotected from light. Light might otherwise disturb the function of theintegrated circuit 12. The second coating 22 may be deposited to theintegrated circuit 12 as a so called glob-top coating. The glob-topcoating consisting of a drop of e.g. specially formulated resin withfilter characteristics such that no light is allowed to pass. Forexample may a black color be used as a filter agent mixed to the resin.

The first coating 16 may have a box-shaped form with two longitudinalside surfaces 18, 19 and a top surface 20. At least one of the sidesurfaces 18, 19 and top surface 20 is a substantially planar surface,such that it can act as an optically surface and lie against the panel2. In FIGS. 4A and 4B, two different alternatives for attaching the PCA15 to the panel 2 are schematically illustrated. In the figures the PCA15 is illustrated in cross-section, where one component in a group 13 isshown, as well as the first coating 16 and the PCB 17. In FIG. 4A, thePCA 15 is placed such that one side surface 18 faces the bottom surface7 of the panel 2. Light from an emitter 4 and light to a detector 5 inthe group 13 may then be transmitted into and out of the panel 2 via theside surface 18 that faces the bottom surface 7. The PCB 17 is hereplaced in a direction to the periphery of the panel 2, in FIG. 4B thePCA 15 is placed such that the top surface 20 faces the bottom surface 7of the panel 2. Light from an emitter 4 and light to a detector 5 in thegroup 13 may then be transmitted into and out of the panel 2 via the topsurface 20 that faces the bottom surface 7. The PCB 17 is here placed ina direction away from the bottom surface 7 of the panel 2.

An adhering means such as glue or double coated adhesive tape may heused between the PCA 15 and the bottom surface 7 of the panel 2 toattach the PCA 15 to the panel 2 as previously illustrated. The adheringmeans may also act as a filler to fill in any irregularities in thesurface of the PCA 15 facing the bottom surface 7 to create a tightattachment, and enable coupling of light from the emitter 4 to the panel2 and from the panel 2 to the detector 5.

The PCA 15 with a plurality of groups 13 of components may be placedaround the periphery of the touch surface 3 or the panel 2, such thatlight from emitters 4 is injected into the panel 2 etc. as previouslyexplained. This is illustrated in FIG. 5, where a touch sensitive system11 comprising a touch sensitive panel 2 defining a touch surface 3 isshown. The system 11 comprises a printed circuit assembly 15 accordingto any of the embodiments previously explained. The printed circuitassembly 15 is attached to the touch sensitive panel 2 along theperiphery of the touch surface 3. The system 11 further comprises acontrol unit 10. The control unit 10 may be connected to one or severalintegrated circuits 12 via a buss 14, daisy chain or via other wiredconnections. The integrated circuits 12 may be connected in series, orin parallel, via the buss 14, daisy chain or other wired connection tothe one or several integrated circuits 12 connoted to the control unit10. The integrated circuits 12 may be arranged to communicate via alocal area network, e.g. a token ring network. The control unit 10 isconfigured to control operation of the components in each group 13. Thecontrol unit 10 is further configured to receive detection data from thedetectors 5 via e.g. one or several signals, and to analyse thedetection data to detect interaction with the touch surface 3. The touchsensitive system 11 is according to one embodiment based on FrustratedTotal Internal Reflection, FTIR.

A screen or display may be integrated with the panel 2, e.g. attached tothe bottom surface 7 of the panel 2. The PCA 15 is then preferablyplaced along the panel 2 outside the extension of the screen or display.If e.g. an 11 inch screen or display is used, the number of emitters 4may be between 40-80, and the number of detectors 5 may be between 40-80to cover the total area of the screen or display. Each integratedcircuit 12 may be connected to 1-4 emitters 4 each, and 1-4 detectors 5.According to one embodiment, only one emitter 4 and one detector 5 isconnected to each integrated circuit 12.

The disclosure also relates to a method for fabricating a printedcircuit assembly 15. The method is here explained with reference to theflowchart in FIG. 6. The method comprises arranging at least one groupof components 13 comprising an emitter 4, a detector 5 and an integratedcircuit, IC, 12 on a printed circuit board, PCB, 17 (A1). The componentsmay be arranged in different manners, for example side by side asillustrated in FIG. 3, or with the integrated circuit 12 in parallelwith the emitter 4 and detector 5 as shown in FIG. 5. Other arrangementsare also feasible, and the illustrated arrangements are only forillustration. The arrangement preferably matches how the componentslater on shall be located in the system 11. For example, the arrangementmay include the distance between each group 13 of components, anddistances between each of the components in a group 13. The PCB 17 isprepared with pads and conductive pathways etc. to enable electricalconnection between the PCB 17 and the components, and between thecomponents. The components are thus arranged such that they match withthe premade soldering spots in a pre-defined pattern. Thus, there is noneed for any intermediate layer between the components and the PCB 17,or any intermediate step of attaching the components to any holdingmeans or other attachments in a package. The components of at least onegroup 13 are then electrically bonded to the PCB 17, and the emitter 4and the detector 5 are electrically connected to the integrated circuit12 via the PCB 17 (A2). This step can be made by e.g. heating thesoldering spots. Hereafter, a first coating 16 is formed covering atleast one of the components, wherein the first coating 16 is made of anoptically transparent material (A3). The optically transparent materialis preferably transparent to infra-red light but blocking to visiblelight. The first coating 16 may be formed to have a box-shaped form asillustrated in FIG. 3, with two longitudinal side surfaces 18, 19 and atop surface 20, wherein at least one of the side surfaces 18, 19 and topsurface 20 is a substantially planar surface. This box-shaped form maybe achieved with a method called “dam and fill” used to encapsulatecomponents. To create a dam, a rectangle of fluid is dispensed aroundthe component. This fluid is typically high viscosity in nature, so onceit is dispensed, it does not flow. Liquid fill encapsulant, i.e. thematerial of the first coating, is then dispensed at high speed over thecomponent and its wires, if any, to encapsulate them. The dam will keepthe fluid in place. Another alternative is to cover the component with abox-like form, and to inject encapsulant into the form. The firstcoating 16 is then cured (A4). The curing step comprises for exampleultraviolet (UV) curing or heat curing. Heat curing may be performed atroom temperature.

The present invention is not limited to the above-described preferredembodiments. Various alternatives, modifications and equivalents may beused. Therefore, the above embodiments should not be taken as limitingthe scope of the invention, which is defined by the appending claims.

1. A printed circuit assembly, comprising: a printed circuit board(PCB); a group of components comprising an emitter, a detector and anintegrated circuit, wherein the components are electrically bonded tothe PCB, the emitter and the detector are electrically connected to theintegrated circuit via the PCB, and the integrated circuit is configuredto control operation of the emitter and the detector in the group; and afirst coating covering at least one of the components in the group,wherein the first coating comprises an optically transparent material.2. The printed circuit assembly according to claim 1, wherein theoptically transparent material is transparent to infra-red light.
 3. Theprinted circuit assembly according to claim 1, wherein the opticallytransparent material is configured to block visible light.
 4. Theprinted circuit assembly according to claim 1, wherein the first coatinghas a box-shaped form with two longitudinal side surfaces and a topsurface, wherein at least one of the side surfaces and top surface is asubstantially planar surface.
 5. The printed circuit assembly accordingto claim 1, wherein at least one of the components in the group iselectrically bonded to the PCB via one or more connections.
 6. Theprinted circuit assembly according to claim 1, wherein the integratedcircuit is covered with a second coating, wherein the second coatingcomprises an optically non-transparent material.
 7. The printed circuitassembly according claim 1, wherein the PCB is a flexible printedcircuit board.
 8. The printed circuit board according to claim 1,wherein the PCB has a longitudinal extension and comprises a pluralityof groups of components, wherein the groups of components are positionedat a distance from each other along the longitudinal extension of thePCB.
 9. A touch sensitive system comprising: a touch sensitive paneldefining a touch surface; a printed circuit assembly according to claim1, wherein the printed circuit assembly is attached to the touchsensitive panel along the periphery of said touch surface; and a controlunit connected to the integrated circuit in one or more groups, andconfigured to control operation of the components in each of said one ormore groups.
 10. The touch sensitive system according to claim 9,wherein said touch sensitive system is based on Frustrated TotalInternal Reflection.
 11. A method for fabricating a printed circuitassembly, comprising: arranging at least one group of componentscomprising an emitter, a detector and an integrated circuit on a printedcircuit board (PCB); bonding the components of the at least one group tothe PCB such that the components are electrically bonded to the PCB andthe emitter and the detector are electrically connected to theintegrated circuit via the PCB; forming a first coating covering atleast one of the components, wherein the first coating is made of anoptically transparent material; and curing the first coating.
 12. Amethod according to claim 11, wherein the step of forming a firstcoating comprises forming a first coating having a box-shaped form withtwo longitudinal side surfaces and a top surface, wherein at least oneof the side surfaces and top surface is a substantially planar surface.13. A method according to claim 11, wherein the curing step compriseseither ultraviolet curing or heat curing.